1. Field of the Invention
This invention generally relates to batteries and their monitoring circuits and particularly to a secondary battery monitoring circuit capable of determining a microcurrent flowing through a secondary battery and to a method for measuring the amount of microcurrent flowing through a secondary battery.
2. Discussion of the Related Art
FIG. 1 shows a battery circuit 25, including a battery condition monitoring circuit, which measures a remaining charge state of a secondary battery. FIG. 1 illustrates the circuit of Japanese Patent Application Toku Kai Hei 9-312172 entitled xe2x80x9cA Battery Pack, Charging Device, and Charging System and Charging Method.xe2x80x9d The FIG. 1 circuit uses a microprocessor or microcontroller to detect battery voltage and charge-and-discharge current. Based on the battery voltage and charge-and-discharge current measurements, the monitoring circuit calculates the battery""s remaining capacity or remaining stored charge. The microcontroller can communicate the battery""s remaining capacity to an external device, such as a portable computer. The external device then is able to display information related to the battery, such as remaining usable battery life.
A battery condition monitoring circuit includes a current monitor circuit 18 and a voltage monitor circuit 19. The battery circuit 25 measures the voltage and the charge-and-discharge current of secondary batteries 1, 1A, 1B and 1C. The battery circuit 25 predicts the battery""s remaining capacity from this information and can transfer that remaining capacity information to an external device. The current monitor circuit 18 and the voltage monitor circuit 19 transfer the measured values to the microcontroller 20.
Voltage detector device 17 detects the level of the secondary battery voltage in comparison to a predetermined battery voltage. Switch circuit 3 prevents the lithium ion secondary battery from discharging below the predetermined voltage. Second switch circuit 4 stops the charging of the battery when the battery voltage reaches a predetermined voltage. Together, the voltage detector device 17 and the second switch circuit 4 prevent a lithium ion secondary battery from overcharging. Microcontroller 20 controls the charging and discharging of the secondary batteries.
Aspects of the present invention provide a battery circuit having a capacitor, current amplifier and a monitor. The capacitor is selectively connectable to a secondary battery. The current amplifier amplifies battery current received from the secondary battery and supplies output current to the capacitor. The monitor circuit, which is coupled to the capacitor, detects a characteristic of the secondary battery indirectly by measuring a voltage of the capacitor or by measuring a capacitor discharge current from the capacitor.
Another aspect of the invention provides a battery circuit having a secondary battery, a current amplifier, and a monitor circuit. The current amplifier amplifies current from the secondary battery and supplies output current to a capacitor, wherein a current path from the secondary battery to the capacitor is selectively connected to charge the capacitor. The monitor circuit, which is coupled to the capacitor, detects a characteristic of the secondary battery indirectly by measuring a voltage of the capacitor with the capacitor disconnected from the secondary battery.
Yet another aspect of the present invention provides a method of measuring a microcurrent flowing from a secondary battery in a battery circuit. According to this method, a first switch is turned ON to connect the secondary battery to a capacitor in common with gate terminals of first and second transistors constituting a current amplifier and a second switch is turned OFF to disconnect drain terminals of the first and second transistors. A terminal voltage of the capacitor is monitored with a capacitor voltage monitor to identify when the terminal voltage of the capacitor increases to a predetermined maximum voltage. Then, the first switch is turned OFF and the second switch is turned ON to connect the capacitor and an external load when the terminal voltage of the capacitor reaches the predetermined maximum voltage. The terminal voltage of the capacitor is monitored with the capacitor voltage monitor to identify when the terminal voltage of the capacitor decreases to a predetermined minimum voltage. Then, the first switch is turned ON and the second switch is turned OFF. The above cycle of steps is repeated.